Method of forming doped regions in a photovoltaic device

ABSTRACT

A surface region of a semiconductor material on a surface of a semiconductor device is doped during its manufacture, by coating the surface region of the semiconductor material with a dielectric material surface layer and locally heating the surface of the semiconductor material in an area to be doped to locally melt the semiconductor material with the melting being performed in the presence of a dopant source. The heating is performed in a controlled manner such that a region of the surface of the semiconductor material in the area to be doped is maintained in a molten state without refreezing for a period of time greater than one microsecond and the dopant from the dopant source is absorbed into the molten semiconductor. The semiconductor device includes a semiconductor material structure in which a junction is formed and may incorporate a multi-layer anti-reflection coating. The anti-reflection coating is located on a light receiving surface of the semiconductor material structure and comprises a thin layer of thermal expansion mismatch correction material having a thermal expansion coefficient less than or equal to that of the semiconductor material, to provide thermal expansion coefficient mismatch correction. An anti-reflection layer is provided having a refractive index and thickness selected to match the semiconductor material structure so as to give good overall antireflection properties to the solar cell.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to PCT Application No.PCT/AU2010/000145, international filing date 11 Feb. 2010, which claimspriority to Australian Application No. 2009900924, filed on 3 Mar. 2009and Australian Application No. 2009900562, filed on 11 Feb. 2009.

INTRODUCTION

The present invention relates generally to the field of photovoltaicsand in particular improvements to lasers and their use in solar cellmanufacture is disclosed. A new anti-reflection coating arrangement isalso described.

BACKGROUND

Laser doping of silicon in localised regions beneath metal contacts hasbeen proposed for more than a decade as a low cost approach forpotentially producing high performance solar cells with selectiveemitters. To date, despite more than a decade of research and problemsolving, devices using laser doping in conjunction with ananti-reflection coating, have not achieved their expected performancedue to defects, junction recombination or shunting arising from thelaser doping process. In particular, defects adjacent to the meltedregions resulting from the thermal expansion mismatch between thesilicon and the overlying antireflection coating (ARC), inadequatemixing of the dopants incorporated into the molten silicon and unwantedablation of the doped silicon are significant problems contributing tothe poor electrical performance of devices using laser doping oflocalised areas beneath the metal contacts.

Further, most solar cells use an anti-reflection coating (ARC) on thesemiconductor surface to reduce the amount of light reflected. The ARCis usually chosen to have the right refractive index and thickness so asto reduce the surface reflection to a minimum. A double layer ARC(DLARC) could also be used whereby the refractive index and thickness ofeach individual layer is chosen to reduce the overall reflection to aminimum, with the theoretical reflection minimum for a DLARC being belowthe theoretical minimum for a single layer ARC (SLARC). Mostcommercially manufactured solar cells use a SLARC as it is too complexand expensive to use a DLARC for the small additional benefits inperformance.

Two problems that can result from using an ARC are: firstly, an ARC maymake it difficult to passivate the semiconductor surface onto which itis deposited, therefore leading to increased recombination and devicedark saturation current; and secondly, many potential ARC materials willhave different thermal expansion coefficients to the semiconductormaterial onto which it is deposited, leading to stressing of thesemiconductor surface with possible corresponding defect generationduring treatment at elevated temperature. To overcome the first, surfacetreatments such as the growth of a thin thermally grown oxide layer topassivate the semiconductor surface prior to the deposition of the muchthicker ARC have been used. With this arrangement the thin passivationlayer does not significantly affect the operation of the ARL depositedover it.

However to date, a suitable solution does not appear to have beenproposed for simultaneously achieving good ARC properties whilesimultaneously providing thermal expansion mismatch correction for theARC and passivating the semiconductor material and surface. In reality,a high performance solar cell technology to be viable commercially needsto be able to use an ARC that performs all three functions whilesimultaneously being able to be deposited in a simple low cost process.

SUMMARY

A method is provided for doping a surface region of a semiconductormaterial on a surface of a semiconductor device during its manufacture,the surface region of the semiconductor material being coated with adielectric material surface layer and the doping being performed in oneor more localised regions on the surface region of the semiconductormaterial, the method comprising locally heating the surface of thesemiconductor material in an area to be doped to locally melt thesemiconductor material, the melting being performed in the presence of adopant source whereby dopant from the dopant source is absorbed into themolten semiconductor and wherein the heating is performed in acontrolled manner such that a region of the surface of the semiconductormaterial in the area to be doped is maintained in a molten state withoutrefreezing for a period of time greater than one microsecond.

The dielectric material coating on the surface layer may perform one ormore of the functions of a surface passivation coating, anantireflection coating, or a plating mask.

Preferably the area to be doped is progressively doped by sequentiallylocally heating regions within the area to be doped. The heating sourcemay be continuously scanned over the surface of the semiconductormaterial such that the region being heated is continuously moving overthe surface, creating a moving melting edge and a molten tail extendingfrom a region of refrozen doped material. The heating source may becontinuously scanned over the surface of the semiconductor material, insuch a manner that a currently heated region overlaps previously heatedregions whereby heating one region contributes heat to an adjacentpreviously heated region, and to which the source is no longer applied,to reduce a rate of cooling of the previously heated region. In one suchapproach the heating source may have a constant output.

In another approach, the heating is applied to discrete regions suchthat a newly heated region will melt and will also contribute heat to anadjacent previously heated region, and to which the source is no longerapplied, to reduce a rate of cooling of the previously heated region.

The intensity of heating may be varied over time between a higher levelused to initially melt a region and a lower level used to maintain themolten state. The level of heating may be decreased after melting aregion until the region being heated overlaps an already molten regionby less than a predetermined percentage. It is also possible to subjecta region to refreezing and remelting to achieve required level ofdoping. However, ideally the remelting should occur no more than 3 timesand preferably no more than once.

The heating may be achieved by scanning one or more laser beams over thesurface of the semiconductor material such that a local region ofirradiance by laser beam creates a melted region and scanning the laserover the surface progressively melts a continuous line of surfacematerial. A continuous wave (cw) laser or a Q-switched laser may be usedto heat the surface of the semiconductor material.

A laser beam may also be scanned over the surface of the semiconductormaterial in such a way that a local region of irradiance by laser beamcreates a melted region and scanning the laser over the surfaceprogressively melts adjacent such regions to form contiguous group ofoverlapping such regions. To achieve this a single Q-switched laser maybe operated in a pulsed operation in which it emits multiple pulses eachshorter than a maximum pulse duration of the Q-switched laser with thepulses irradiating overlapping regions of the semiconductor so as tosimulate the effects of a cw laser.

In this arrangement the repetition period of the pulses will typicallybe less than 0.02 μs. The repetition period of the pulses may beconstant to simulates an output of a constant irradiance cw laser or maybe variable to simulate a variable irradiance cw laser.

Where a laser operated to have an output providing substantiallyconstant level of irradiance on the surface being heated, or pulsed at arate to simulate such an output, the scanning speed may then be chosento melt the surface and to maintain a given point on the surface of thesemiconductor material in a molten state for a period of at least 1microsecond but no more than 10 microseconds.

In another approach, an average irradiance level of one or more laserbeams impinging on a surface to locally heat the surface may be variedwith time between at least a high average irradiance level periodicallyapplied for a predetermined time period t1 to cause melting of unmeltedsemiconductor material on the surface of the semiconductor device withinthe time period t1 and a lower average irradiance level applied for afurther time period t2 after the time period t1, the lower averageirradiance level maintaining the melted semiconductor material in amolten state for the time period t2.

With a cw laser, the irradiance level may also be varied cyclically insynchronisation with the scanning speed such that the average irradiancelevel of the laser increases during a melting period and then decreasesuntil the beam has moved over the surface sufficiently for less than apredetermined percentage of the beam to be still exposing moltensemiconductor material. The period of the cyclic power variation maycorrespond to the beam width divided by the scanning speed and thehigher output period may be 50% of the cycle or less.

It is desirable that during the lower irradiance period t2 thetemperature of the molten semiconductor material is kept between 1414°C. and 3265° C. for at least one microsecond and preferably for 2-10microseconds. The irradiance level of the laser may be varied by varyingthe output power of the laser, by varying the focus of the laser beam tospread the beam during the lower irradiance period t2, or when aQ-switched laser is used, by producing pulses in which the output varieswith time such that each pulse has a high irradiance portion and a lowerirradiance portion.

When variable irradiance is provided using a Q-switched laser whichproduces output pulses having variable power within the duration of thepulse, a first period of the pulse may have a power level which meltsirradiated semiconductor material and a second period may have an outputpower that maintains irradiated melted semiconductor material in themolten state. During the second period the laser will preferably have anoutput power that does not ablate molten semiconductor material.

In one possible approach, the variable average irradiance is providedusing a single Q-switched laser which is operated in a pulsed operationin which it emits multiple pulses each shorter than a maximum pulseduration of the Q-switched laser with the pulses irradiating overlappingregions of the semiconductor, the repetition period of the pulses beingless than 0.02 μs and the repetition period being varied to simulate avariable output cw laser which produces a cyclic variable output. Ahigher average irradiance level may be produced by providing moreclosely spaced pulses the cyclic output having an average irradiancelevel during a first period of each cycle which melts irradiatedsemiconductor material and an average irradiance level during a secondperiod of each cycle that maintains irradiated melted semiconductormaterial in the molten state. During the second period the laser willalso preferably have an average output irradiance level that does notablate molten semiconductor material within the period of illumination.Ideally the average output irradiance level during the second periodwill be such that it could be applied indefinitely without ablatingmolten semiconductor material.

The variable irradiance may also be provided using a cw laser whichproduces a cyclic variable output having a power level during a firstperiod of each cycle which melts irradiated semiconductor material and apower level during a second period of each cycle that maintainsirradiated melted semiconductor material in the molten state. Againduring the second period the laser preferably has an output power thatdoes not ablate molten semiconductor material.

More than one laser may be used to heat the semiconductor surface, inwhich case one laser may be used to melt the surface and a second lasermay be used to maintain the temperature for a period sufficient to allowthe dopant to be absorbed and mixed. Q-switched lasers may be used toheat the surface, or a first, Q-switched laser may be used to melt thesemiconductor material with a pulse rate and scanning speed selected toallow overlapping exposed areas for consecutive pulses. In thisarrangement, the pulses of the Q-switched laser may be of an intensitywhereby the semiconductor material is melted with a single laser pulseand have a duration of less than 10% of the pulse repetition period.Each laser pulse may be overlapped with the previous pulse by 10-50% ofthe exposure area of the pulses. The second laser in this arrangement isonly used to ensure that the semiconductor material remains moltenbetween the pulses of the first laser for the required period of timeand may be a cw laser operated with a constant output.

However, the second laser may also be a Q-switched laser, in which casethe second laser may operate to produce output pulses in periods betweenthe pulses of the first laser and at a lower irradiance than the firstlaser to extend the time during which semiconductor material melted bythe first laser remains molten. The second laser may be operated in apulsed operation in which it emits multiple pulses each shorter than amaximum pulse duration of the Q-switched laser with the pulsesirradiating overlapping regions of the semiconductor, the repetitionperiod of the pulses being less than 0.02 μs and the repetition periodbeing constant, to simulate a constant output cw laser at least duringthe period between the pulses of the first laser.

The area to be doped may comprise the entire surface of thesemiconductor device to create a surface emitter layer. However, moretypically this general method is used to dope a part of an entiresurface where contacts are to be formed on an illuminated surface of asolar cell. The method is typically used with silicon material althoughit is equally applicable to other semiconductor materials.

The period within which the surface of the silicon material may bemaintained in a molten state when applying the above methods will bepreferably a period which permits substantially uniform distribution ofthe dopant throughout the molten semiconductor material. Alternativelythe period within which the surface of the silicon material may bemaintained in a molten state may be a period which permits all of themolten region to achieve the same overall dopant polarity.

The laser doping process for a silicon wafer involves melting localisedsurface regions of the wafer in the presence of either n-type or p-typedopants, so that the dopants are incorporated into the molten region.The dopants may be included within a surface dielectric layer, they maybe applied as a coating on top of or below the dielectric layer (alsopotentially the antireflection coating), they may be present in thesilicon at interstitial sites in an unactivated state whereby they areabsorbed into the silicon structure (or activated) when the siliconcrystallizes in the melting and refreezing process or they may beapplied to the region in gaseous or liquid form whilst the silicon ismolten.

A multi-layer anti-reflection coating for a solar cell is providedwherein the solar cell includes a semiconductor material structure inwhich a junction is formed, the anti-reflection coating being located ona light receiving surface of the semiconductor material structure andcomprising a thin layer of thermal expansion mismatch correctionmaterial having a thermal expansion coefficient less than or equal tothat of the semiconductor material, to provide thermal expansioncoefficient mismatch correction; and an anti-reflection layer having arefractive index and thickness selected to match the semiconductormaterial structure so as to give good overall antireflection propertiesto the solar cell.

A method of fabricating a multi-layer anti-reflection coating for asolar cell is also provided wherein the solar cell includes asemiconductor material structure in which a junction is formed, theanti-reflection coating being formed on a light receiving surface of thesemiconductor material structure and the method comprising forming athin layer of thermal expansion mismatch correction material having athermal expansion coefficient less than or equal to that of thesemiconductor material, to provide thermal expansion coefficientmismatch correction; and forming an anti-reflection layer under or overthe thermal expansion mismatch correction material, the anti-reflectionlayer being selected to have a refractive index and thickness whichmatches the anti-reflection layer to the semiconductor materialstructure so as to give good overall antireflection properties to thesolar cell.

The proposed multilayer anti-reflection coating is preferably applied toa crystalline (including multicrystalline) silicon based device but mayalso be applied to devices based on other semiconductor types in whichcase the thermal expansion mismatch correction material will be selectedto match the thermal co-efficient of expansion of the particularsemiconductor material.

The thermal expansion mismatch correction material will be formed, inone embodiment, after surface passivation (if applied) and before theanti-reflection layer. However in another proposed embodiment aconventional SLARC is applied followed by a thermal expansion mismatchcorrection material layer of similar or greater thickness to the SLARCand having a refractive index matched to a subsequently formedencapsulation layer.

When applied before the anti-reflection layer, the thermal expansionmismatch correction material layer is preferably at least as thick asthe passivation layer (if used) but not as thick as the anti-reflectionlayer. With PECVD it is possible to tailor this layer in terms of havinga graded composition and refractive index to further aid with theoptimisation. The inclusion of some nitrogen allows silicon oxynitridesof virtually any refractive index from below 1.5 to above 2 to beachieved.

When the thermal expansion mismatch correction material layer is appliedafter the anti-reflection layer, the DLARC is preferably quite standard(to achieve surface passivation and good antireflection properties), andthe expansion mismatch correction material layer is formed with athermal expansion coefficient less than silicon but with opticalproperties matched to the encapsulant (i.e. refractive index of about1.5) so as not to degrade the antireflection properties of the solarcell when encapsulated. This additional layer may be of silicon dioxideor silicon oxynitride and needs to be quite thick (at least as thick asthe silicon nitride anti-reflection layer) to provide adequate thermalexpansion mismatch correction.

The front surface region of the semiconductor material structure may bepassivated by a surface treatment of its light receiving surface. Thepassivation treatment may comprise a surface diffusion of thesemiconductor material or a surface coating such as a very thindielectric layer. Passivation may also be achieved by selecting athermal expansion mismatch correction material having or modified tohave passivation qualities.

For a crystalline silicon based solar cell the passivation treatment maycomprise a thin silicon nitride layer in the range of 10-200 angstromthick, the thermal expansion mismatch correction layer may comprise alayer of silicon dioxide or silicon oxynitride in the range of 100-300angstroms thick and the anti-reflection layer may comprise a layer ofsilicon nitride in the range of 300-800 angstroms.

The multiple layer coating may also act as a dopant source during afront surface laser doping process by incorporating dopants into one ofthe layers being already used for providing one or more of the functionsabove. Alternatively an additional dopant source layer can be formedwith refractive index of approximately 1.5 so as to optically match tothe material to be used as encapsulant for the solar cells during moduleformation.

In the case of the expansion mismatch correction material layer beingformed over the SLARC, the expansion mismatch correction material layermay simultaneously also be a dopant source for doping of the heavilydoped regions of a contact structure. In this case, the additionalexpansion mismatch correction material/dopant source layer may beremoved prior to encapsulation if desired. The anti-reflection layer inthis case may be a layer of silicon nitride in the range of 300-800angstroms, and the thermal expansion mismatch correction layer may be alayer of silicon dioxide or silicon oxynitride in the range of 300angstroms to 1 micron thick with a refractive index in the range of1.5-1.6. This arrangement provides a new option where an additionalmaterial is added on top specifically for the high temperaturetreatments (such as laser doping) to give the thermal expansion mismatchcorrection but can then be optionally removed prior to cell completionso as not to interfere with the optical properties.

A wide range of deposition approaches such as PECVD, sputtering orevaporation may be used to apply the passivation/thermal expansionmismatch correction/anti-refection layers. A wide range in the number oflayers and their dimensions and refractive indices could also be used.For example a portion of the deposition of the anti-reflection coatingmay be applied for the purpose of providing the thermal expansionmismatch correction, in which case a portion may be applied for thepurpose of passivating the semiconductor surface and/or bulk while aportion is deposited to provide the overall anti-reflection coating withthe desired optical properties of reduced reflection.

The proposed multilayer thermal expansion mismatch correction layer maybe applied to a non light-receiving surface which therefore eliminatesthe need for the overlying anti-reflection layer.

The proposed multilayer anti-reflection coating may be applied to asurface so that the anti-reflection coating can also perform additionalfunctions such as acting as a plating mask or a diffusion mask. Whereadditional functions such as these need to also be performed, it isoften feasible to incorporate the necessary properties into one of theexisting layers by appropriately varying the chemistry of the layer orits thickness or alternatively by incorporating one or more additionallayers to provide the additional functions.

The proposed multilayer anti-reflection coating may be used with a rangeof surface structures including textured surfaces and the density ofdefects generated due to thermal expansion mismatch can be dependentupon this surface geometry. Never-the-less, regardless of the surfacegeometry, the multilayer anti-reflection coating can be applied so as tosignificantly reduce the generation of defects during thermal processes.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theaccompany drawings in which:

FIG. 1 illustrates a Solar cell with a selective emitter structure andmetal contact formed in self-aligned method following the laser dopingof the heavily doped regions beneath the metal contact. The manufactureof this solar cell may also incorporate one of the improved laseroperation methods described herein;

FIG. 2 schematically illustrates the major components of an opticallypumped solid-state laser oscillator;

FIG. 3 is a photograph of an Optical Cavity of a Quantronix™ series 100Nd:YAG laser;

FIG. 4 graphically illustrates Cavity losses, gain and laser energyoutput w.r.t. time for a Q-switched laser;

FIG. 5 graphically illustrates an example of a laser beam power versustime profile for laser pulses to improve the laser doping of silicon(times not drawn to scale);

FIG. 6 graphically illustrates the heating effect of a laser pulse onsilicon;

FIG. 7 graphically illustrates two power versus time profiles A and Bfor the laser output which are equivalent from the silicon's perspectiveprovided t<<1 microsecond and provided the cumulative energy deliveredto the silicon is the same in both cases;

FIG. 8 graphically illustrates a Power versus time profile (curve A)which in curve B is equivalently synthesized by a sequence of short highenergy pulses with time varying separation;

FIG. 9 graphically illustrates a variation of electron density inexcited states N and photon flux during the transient behaviour thatfollows briefly after introducing loss into an optical cavity of a laserto interrupt the steady state continuous wave operation of the laser;

FIG. 10 graphically illustrates one example of how laser output powerfor a continuous wave laser may be varied in conjunction with scanningspeed and beam diameter to achieve a desirable and incident powerprofile at a point over which the laser is scanned;

FIG. 11 schematically illustrates the effect on a silicon surface undera scanned laser beam using a proposed laser control method;

FIG. 12 schematically illustrates the arrangement of a laser and liquidjet when used to process a surface of a target; and

FIG. 13 schematically illustrates a laser operated to heat a target in agaseous environment to process a surface of a target.

FABRICATION USING CONTROLLED HEATING

By way of example, and with reference to FIG. 1, a suitable fabricationsequence for the formation of a silicon solar cell is as follows:

-   -   1. Isotropic texturing 12 of the front (or light receiving)        surface of the p-type wafer 11;    -   2. front surface diffusion of n-type dopant 13;    -   3. edge junction isolation/psg removal;    -   4. ARC deposition on the front surface by PECVD;        -   a. 100 angstroms of hydrogen rich silicon nitride            specifically for surface passivation 14;        -   c. 600 angstroms of silicon nitride 16 of refractive index            2.0-2.1;        -   d. dopant containing layer 17;    -   5. screen-print rear (non-light receiving) surface with        aluminium for rear contact 18;    -   6. fire rear surface to sinter rear contacts 18 and form back        surface field 19 by formation of aluminium/silicon alloy and        liquid phase epitaxy;    -   7. laser doping of silicon in localised regions to form heavily        doped (n+) regions 22 for formation of self-aligned front        surface metal contacts;    -   8. Plating a layer of Nickel 23 over the laser doped n+ regions        22 for the front surface contacts;    -   9. sintering of Nickel 23;    -   10. plating of layers of Copper 24 and Tin 25 (or Silver) over        the Nickel 23;

The above processing sequence produces the high performance solar cellstructure of FIG. 1 with a selective emitter that provides heavy dopingof the silicon directly beneath the metal contacts. The presentlyproposed controlled laser heating method may be used in conjunction withthis manufacturing sequence to reduce the formation of defects in theregion of the contacts.

Improved heating regimes can be effected by a uniquely designed laserQ-switching arrangement or by a scanning continuous wave laser withappropriate power level, to melt the silicon for adequate duration tofacilitate dopant mixing while simultaneously avoiding unnecessarythermal cycling of the melted regions or ablation of the doped silicon.Q-switched laser systems or directly applied continuous wave lasersoperated in the conventional manner are unable to heat and melt thesilicon in the required way.

FIG. 2 shows the major components of an optically pumped solid-statelaser oscillator 31. A pump cavity 32 contains a laser rod 33 and a pumplamp 34. The laser rod 33 is a semiconductor material such as aNeodinium:YAG crystal, which when illuminated by the pump lamp 34, leadsto the excitation of large numbers of electrons N into high energylevels (inversion). Continued illumination of the laser rod 33 by thelamp 34 causes N to increase until the generation rate of excitingadditional electrons is balanced by the spontaneous recombination ratefor the excited electrons which occurs predominantly by radiativerecombination at 1.064 micron wavelength light in Nd:YAG laser rods or532 nm if operating in frequency doubled mode. The basis of lasingaction is a process known as “stimulated emission” where therecombination of an excited electron is triggered by an incident photon.In this process, the emitted photon is indistinguishable from theincident photon triggering the stimulated emission. It has the samewavelength of 1.064 microns (or 532 nm when frequency doubled), travelsin the identical direction and has the same phase. This process can beused in conjunction with the mirrors 35 and 36 shown in FIG. 2 to forman optical resonator with laser oscillations. The mirrors 35, 36 aremounted parallel to each other and perpendicular to the axis of thelaser rod 33, so that if a spontaneously emitted photon fortuitouslytravels in line with the laser rod's axis, it is reflected back into thelaser crystal by the respective mirror 35 or 36, providing theopportunity for stimulated emission and the generation of multiplephotons all of the same wavelength, phase and direction. This gives riseto the potential for amplification if the gain is above unity, leadingto an increase in the number of photons within the optical cavity 11.These are then reflected by the other mirror 36 or 35, again back alongthe axis 17 of the laser rod and into the laser crystal. These laseroscillations continue to increase the photon flux for as long as thegain remains above unity, where the gain is determined by the number ofelectrons in their excited (inverted) state, the stimulated emissionprocess and the cross-section of the laser rod volume.

Steady State Continuous Wave Laser Operation

In steady state, the gain will be unity with the rate at which newphotons are emitted by the laser crystal exactly balancing the totaloptical losses from the optical cavity. The major optical loss from thecavity is via the mirror 35 at the output end of the laser which isdeliberately partly transmissive to allow the laser beam to escape. Thismode of laser operation is known as continuous wave since the laser beamis delivered continuously from the optical resonator.

For a given laser rod 33, optical cavity 32 and pumping lamp power, thissteady state operation of the laser corresponds to a “threshold”population inversion density of electrons in their excited state N_(th).For N>N_(th), the gain will exceed unity leading to an increase in thelaser oscillations and photon flux which will in turn cause an increasein the stimulated emission and a corresponding lowering of the number ofelectrons in their excited state N. Similarly, if N<N_(th), the gain isbelow unity and less than the steady state value of stimulated emissionoccurs, leading to the generation rate of excited electrons due to thepumping lamp exceeding the recombination rate determined by thestimulated emission, and so N increases.

Q-Switched Laser Operation

Q-switching is a mode of laser operation extensively used for thegeneration of high power pulses that makes it easier to melt the siliconduring the laser doping process. Energy is effectively accumulated andstored for a period of time in the laser crystal and then used to form avery high energy laser pulse that is emitted from the laser in anextremely short period of time, therefore providing extremely high powerdensities. The energy is accumulated and stored in the laser crystal bydeliberately creating optical losses within the optical cavity to retardstimulated emission and therefore prevent lasing action. This allows thegeneration rate of excited electrons to greatly exceed the recombinationrate, therefore allowing N to greatly exceed N_(th). This state of thelaser crystal is known as “population inversion”. Subsequent eliminationof the optical loss from the optical cavity allows stimulated emissionto again occur and the photon flux in the optical cavity increasesexponentially until N is sufficiently depleted to retard significantfurther radiative recombination. This leads to the emission of a veryhigh energy pulse from the laser, following which few electrons remainin their excited state with N falling to way below N_(th) and the gainfalls to well below unity.

Introducing the optical loss into the optical resonator is oftenreferred to as lowering the quality factor Q, where Q is defined as theratio of the energy stored in the optical cavity to the energy loss percycle. FIG. 3 shows a photo of the optical cavity of a Nd:YAG series 100Quantronix™ laser 41, showing a flooded optical pump cavity 42,enclosing a Nd:YAG crystal assembly 43 and krypton arc pumping lampassembly 44. The beam passes from the crystal assembly to one mirrorassembly 46 via a beam tube and bellows 48 mounted on rail 49. Mirroradjustments 51 provide alignment of the beam axis to the mirror 46.Q-switch Bragg angle adjustment 52 a mode selector 53 and an intercavitysafety shutter 54 are provided at the output end of the optical path. Inthis case, the Q-switch 55 is a prism through which the laser beampasses. Electrodes mounted on the prism allow a high powered RF signalto be applied to the prism which causes the laser beam to deviate andtherefore create optical losses. FIG. 4 shows how control of the qualityfactor can be used to generate the high energy pulses and how N changesthroughout. When considering laser doping, it turns out that a severelimitation of Q-switched lasers is the fact that following thegeneration of each high energy pulse, N falls to such low values (wellbelow N_(th)) due to stimulation emission from the very high photon fluxin the optical cavity, that minimal lasing action is feasible until Nhas returned to well above Nth. This takes too long from the perspectiveof laser doping since the molten silicon being doped re-solidifiesduring this period.

Laser Doping Process

The laser doping process for a silicon wafer involves melting localisedsurface regions of the wafer in the presence of either n-type or p-typedopants, so that the dopants are incorporated into the molten region.Referring to FIG. 1, this facilitates the formation of a selectiveemitter structure with the heavily doped regions 22 self-aligned to theoverlying metal contact 23, 24, 25. The dopants can be included within asurface dielectric layer 17, be applied as a coating on top of or belowthe dielectric layer (also potentially the antireflection coating 16),they may be present in the silicon in an unactivated state whereby theyare absorbed into the silicon structure (or activated) by the meltingand refreezing process or they may be applied to the region in gaseousor liquid form whilst the silicon is molten (described below withreference to FIGS. 12 and 13). Referring to the FIG. 1 example, to uselaser doping in conjunction with subsequently forming self aligned metalcontacts 23, 24, 25 onto the highly doped laser melted regions 22, thesilicon surface is coated with a dielectric layer 17 that protects theunmelted regions from the subsequent metal contact formation process astaught by Wenham and Green, U.S. Pat. No. 6,429,037. The laser dopingprocess automatically destroys the overlying dielectric layer in thelaser doped regions, therefore exposing the silicon surface forsubsequent metal contact formation which can be done in a self-alignedprocess such as via metal plating. The dielectric layer or layers caninclude an antireflection coating 16, surface passivation layer 14,dopant source 17, hydrogen source (not shown) for surface and/or grainboundary and/or defect passivation, protection layer (also not shown)for the silicon surface and/or plating mask, or one or more layers whichpotentially in combination or singly perform one or more of thesefunctions.

The dopant source may also be incorporated within the silicon itself,rather than in a separate layer or coating. In other words, the lasermay be used to locally melt the silicon that is already loaded withdopant such that the melting and refreezing process causes the freedopants (commonly referred to as interstitial atoms which areelectrically inactive dopants that are not bonded normally within thesilicon lattice) to be absorbed into the crystalline silicon structure(lattice) and redistributed from original location. For example when ann-type dopant is thermally diffused into the surface of the silicon inan emitter forming step, many more n-type dopant atoms may be diffusedinto the silicon than actually become electrically active. Laser meltingcan then be used to allow these extra dopant atoms to redistributethemselves and become active within the silicon to form more heavilydoped contact regions.

The number of inactive dopants that will be present within the silicon(e.g. within the diffused n-type emitter) is determined by the way inwhich the diffusion is done. Often when diffusing dopant into a surfaceit will be done it in a way that attempts to keep the surfaceconcentration of the dopant (e.g phosphorus (P)) below the solidsolubility of the dopant in Silicon, for the particular temperature atwhich the processing is being performed, to avoid the inclusion of toomany inactive dopants. One way of avoiding excessive dopant atoms in thesilicon, for example, is by diffusing through a silicon dioxide layer(most common approach) although another very common approach is simplyto reduce the concentration of the dopant source.

By deliberately allowing the surface dopant concentration (e.g.phosphorous) to go above the solid solubility of the dopant in silicon,whereby large numbers of inactive dopants are incorporated into thesurface, these inactive dopants can become the source of dopants for thelaser doping process. Typically the emitter will be formed with a sheetresistivity in the range of 80-200 ohms per square. By incorporating alarge number of inactive dopants, the sheet resistivity can be reducedby at least a factor of two in the areas treated by the laser comparedto the areas not treated by the laser. For example, if the emitter isformed with a preferred emitter sheet resistivity of 100 ohms persquare, this can be made to drop to about 30-40 ohms per square in areasmelted by the laser. This level of sheet resistivity is sufficient forgood performance but optimisation of the process may provide even betterresults.

Problem with Conventional Q-Switched or Continuous Wave Lasers

A particular challenge with the above process arises due to limitationsimposed by conventional Q-switched or continuous wave (cw) lasers insteady state. In a continuous wave laser in steady state, it isdifficult to melt the silicon and then hold it molten for sufficientduration to ensure mixing of the dopants and then stop the energy flowfrom the laser before ablating some of the silicon. This is because ingeneral if there is sufficient energy in the beam to melt the siliconwithin a reasonable period of time (at approximately 1400 degreesCelsius), then during the subsequent 2-10 microseconds that the siliconneeds to remain molten for the dopant mixing, the constant delivery ofthe laser energy from the cw beam into the molten region during thisperiod is likely to heat the silicon to above its vaporisationtemperature T_(v) as shown in FIG. 6. In FIG. 6, the unacceptablescenario is shown in the dotted curve where the silicon is heated towell above the vaporisation temperature T_(v) when the cw laser insteady state with energy E_(ss) is applied to a specific location. Toavoid the unacceptable ablation of silicon, either the energy E_(ss) hasto be reduced sufficiently so that the highest temperature T_(u) thatthe silicon reaches is still below the ablation temperature T_(v), orelse the application of the laser beam to this location has to cease bytime t₂ as shown by the solid curve in FIG. 6 so the silicon only heatsto temperature T₂ before being allowed to cool. It is difficult with acw laser in steady state to keep applying and then ceasing theapplication of the laser beam in this way because of problems such astransient effects in the laser output and the laser operation. Even ifthe cw laser could be applied and stopped in this way, the energy levelE_(ss) and timing would also have to be such that the time periodbetween t₁ (the time at which the silicon melts) and t₂ (when the laserbeam ceases) would need to be at least 2 microseconds or else thesilicon will not remain molten long enough for the dopants to properlymix. Pseudo continuous wave lasers which typically work at pulsefrequencies well above 10 MHz, have the same problem whereby the energypulses are so close together that from the silicon's perspective, itbehaves similarly to a cw laser since the silicon does not havesufficient time between the pulses to significantly change temperature.

Conventional Q-switched lasers also cause problems with the laser dopingprocess. This is because the pulses are too short and too far apart.Pulse durations are usually well below 1 microsecond, leading to thesilicon remaining molten for typically no more than a microsecond, whichis insufficient for adequate dopant mixing. To compensate for this, manyoverlapping pulses are therefore typically needed at each location tomelt the silicon enough times so that the cumulative effect providessufficient time in the molten state for adequate dopant mixing. Thishowever introduces another problem relating to the thermal cycling. Theduration between pulses in these Q-switched lasers is sufficiently long(usually at least several microseconds) that the silicon resolidifiesbetween pulses. This cycling creates significant stress on the siliconleading to defect generation, particularly in the regions immediatelyadjacent to the areas melted, exacerbated by the thermal expansioncoefficient mismatch between the silicon and the antireflection coatingwhich in general is silicon nitride. Each additional pulse at eachlocation reheats the solid silicon immediately next to the molten regionto a temperature close to the melting temperature for silicon. At thesetemperatures, the higher thermal expansion coefficient of the siliconnitride (or any other layer with thermal expansion coefficient abovethat of silicon), leads to it placing the silicon surface under tension.Under these conditions, the silicon is particularly weak and vulnerableto defect generation. The more of these cycles the silicon sustains, theworse apparently is the defect generation. Consequently there is afundamental weakness and trade-off with this type of laser wherebyinsufficient pulses at each location leads to improper dopant mixing butnot too many defects, while additional pulses facilitates better dopantmixing but increased defect generation.

Considerable research has been conducted in various institutions and/orcompanies in recent years addressing this problem associated with laserdoping. Unfortunately, the solutions found have not been practical forthe commercial production of laser doped solar cells. One solution hasbeen to replace the silicon nitride ARC with a silicon dioxide layerthat has lower thermal expansion coefficient than silicon. The use ofsuch a layer when heated to the vicinity of the melting temperature forsilicon, places the silicon surface under compression, thereforeavoiding defect generation. Good electrical performance has beenachieved from devices fabricated with this approach with the achievementof fill factors above 80% indicating that junction recombination due todefects has been reduced to insignificant levels. This approach is nothowever practical for commercial devices, firstly because the lowerquality of commercial grade substrates relies heavily on hydrogenationfrom the silicon nitride layer to passivate defects and grainboundaries, secondly because silicon dioxide layer has too low arefractive index to be effective as an antireflection coating, andthirdly, the thermal growth process for such oxide layers is notpractical for commercial devices.

A modified approach of using a thin interfacial oxide layer sandwichedbetween the silicon surface and the silicon nitride ARC has solved someof the above limitations while still facilitating the avoidance of thedefect generation and hence the achievement of high fill factors. Theoptical performance of the ARC combination was only marginally belowthat of the straight silicon nitride ARC, while the practicalities ofgrowing a thin oxide layer (150-200 angstroms) industrially are greatlysuperior to having to grow an oxide layer of ARC thickness (1100angstroms). The main drawback however, in addition to the increased costand complexity of having to grow the thin oxide layer, is the inabilityto hydrogenate (passivate) defects and grain boundaries through such anoxide layer.

Improved Regime for Heating and Melting the Silicon

To improve the quality of the laser doping process as part of forming aself-aligned metal contact, the silicon needs to be heated in adifferent manner to the way it has been previously done withconventional laser systems. Following melting to incorporate the dopantsinto the silicon, heat needs to be continuously applied to the moltenregion for time t₂ (see FIG. 5) so as to keep the temperatureapproximately constant for at least one microsecond and preferably for2-10 microseconds. This is to allow adequate time for the dopants toredistribute uniformly throughout the molten region while simultaneouslykeeping the molten volume approximately constant which is not possibleif the temperature is changing. To achieve this, the power received fromthe laser in a given location needs to vary with time as shown in FIG. 5where the initial high powered region of the pulse of duration t₁ isrequired to quickly melt the silicon while the lower powered tail needsto be of the right duration t₂ and power level so as to keep the moltenregion at approximately constant temperature for the required duration.Of particular importance is that the silicon remains molten for up to2-10 microseconds to ensure adequate quantities of dopants areincorporated into the molten region and that these dopants are properlymixed within the molten silicon. This process simultaneously exposes theheavily doped silicon surface by destroying the overlying dielectriclayer (usually silicon nitride). This facilitates the subsequentformation of a metal contact such as through the direct plating of metalto these heavily doped regions. This process can therefore lead to theformation of a selective emitter whereby the low area metal contactformed via the laser doping process is automatically self aligned to theheavily doped regions as shown in FIG. 1.

A solution for laser doping of silicon is to deliver the laser pulsewith its energy as a function of time as shown in FIG. 5, whereby theinitial peak in energy is for sufficient duration t₁ to heat the siliconto its melting point, while the subsequent and much longer part of thepulse is at a much lower energy level that continues for severalmicroseconds and is at an energy intensity that keeps the molten siliconat approximately a constant temperature. At the end of the pulse, thelaser energy falls for period t₃, allowing the silicon to cool andsolidify until the next laser pulse arrives. Ideally, the frequency ofthese pulses F_(pulses), in conjunction with the scanning speed of thelaser beam V_(scan), are chosen so that the distance traveled by thescanning beam in the time 1/(F_(pulses)) is within the range of 50-100%of the beam diameter. This ensures the laser doping forms a continuousline but prevents excessive overlap between adjacent pulses so that noneof the silicon is melted more than twice so as to avoid excessivethermal cycling. For example, if V_(scan) is 1 m/s and the frequency 100kHz, then the laser beam moves 10 microns relative to the siliconsurface in the time between the commencement of juxtaposed pulses. Thisrepresents approximately 67% of the typical beam diameter which isapproximately 15 microns.

One approach to achieving this outcome is to superimpose two laser beamson top of each other whereby one is Q-switched at the desired pulsefrequency with a power per pulse close to that required to melt thesilicon while the second laser operates on continuous wave steady statemode, providing an energy level appropriate to keep the molten siliconat approximately a constant temperature. The latter laser power is toolow to melt the silicon in the time each location is exposed to thelaser (or if it does melt the silicon, the time taken is too long sothat the silicon does not then remain molten for long enough). Howeversuperimposing the Q-switched laser pulses from another laser (withindependent optical cavity) allows the silicon to be quickly heated tothe right temperature for melting, following which the low powered cwlaser provides sufficient energy to keep the silicon molten. In thisregime though, some overlap between juxtaposed laser pulses is necessaryas the beam is scanned across the wafer surface to ensure continuity inthe melted and doped regions. This can be a problem since the meltedsilicon from one pulse still remains molten when the next pulse arrivesdue to the low powered laser keeping its temperature constant, withsubsequent overlap with the next Q-switched pulse therefore in danger ofcausing the already molten silicon to reach its vaporisationtemperature. It seems a small amount of ablation is therefore inevitablewith this approach although it is still a significant improvement overlaser doping done with conventional Q-switched or continuous wavelasers. Ideally, the low powered cw laser needs to have its powerreduced or stopped briefly before the next Q-switched pulse arrives soas to allow the silicon to cool somewhat. Although laser systems can intheory be configured to work in this way, in reality, difficulties inaligning the two lasers make it unreliable at best and unworkable atworst.

Another approach involves controlling the Q of a Q-switched laser whichcan in theory facilitate the formation of laser pulses with power as afunction of time as shown in FIG. 5. In normal Q-switched lasers this isnot an existing capability since the loss introduced into the opticalcavity is binary in nature with only two discrete levels, one causingthe Q to be high (low loss) so the laser gain is high and one causingthe Q to be low (high loss) so that the gain is low. With a speciallydesigned arrangement for controlling the Q of the cavity with infinitelyvariable Q values, it becomes theoretically possible to construct powerversus time profiles for the energy pulses as shown in FIG. 5 where theinitial high energy region is of the right peak power level and durationto melt the silicon, the tail is also of the right power level andduration so as to keep the molten region at approximately a constanttemperature for the time needed for adequate dopant mixing, followed bya brief period at much lower energy level that allows the silicon tocool sufficiently before the next laser pulse arrives. The overalltiming and frequency of the pulses is chosen so as to give the desiredlevel of overlap between juxtaposed pulses.

Another approach for achieving the equivalent of the previous approachabove is through synthesizing the power versus time profile with asequence of closely spaced pulses (of much lower energy content thannormal Q-switched pulses) which when “filtered” to remove the highestharmonics produces the power versus time profile of FIG. 5. This can bedone using a more conventional Q-switching arrangement of onlyintroducing loss into the optical cavity in a binary fashion whereby theloss level is only either high or low, but is applied in a morecontrolled fashion with regard to the length of time for which the lossis applied. When a Q-switched pulse is to be generated, the deliberatelyapplied loss to the optical cavity is removed, therefore allowingstimulated emission and the generation of a high energy pulse. If theloss is reapplied quickly enough partway through the generation of apulse, the number of electrons N in the excited state can remain in thevicinity of Nth or above, therefore making it feasible to generateanother pulse almost immediately. A series of much smaller pulses over alonger period of time can therefore be generated instead of a singleshort high energy pulse following which N falls to negligible values.When applying such a sequence of energy pulses to silicon, provided theperiod between pulses is short enough (well under a nanosecond), thelimited thermal conductivity of the silicon means that it is simply theamount of energy delivered to the silicon by the laser pulsescollectively rather than the actual form it takes (i.e. power versustime relationship) that is important. For example, from the perspectiveof the silicon being melted, the two forms of laser output shown in FIG.7 are equivalent provided “t” is well under a nanosecond. By virtue ofthis “filtering” function performed by the silicon, this makes itpossible to synthesize virtually any desired laser output as a functionof a large number of small pulses closely spaced in time.

To achieve control of the size of each of the small pulses describedabove, feedback linked to the photon flux in the optical cavity can beused so that when this photon flux reaches a specific level during theformation of a Q-switched pulse, loss can be reintroduced into theoptical cavity to prevent further stimulated emission and thereforepreventing the formation of the remainder of the high energy pulse thatwould otherwise eventuate. Referring to FIG. 12, one way this feedbackcan be applied is via the use of a slightly transmissive mirror on thenon-output end of the optical cavity with a photosensitive device suchas a solar cell able to measure the intensity of the light escapingthrough the mirror which is in turn proportional to the photon flux inthe optical cavity. The measurement of this light intensity can be usedto determine the time at which the loss should be reapplied to theoptical cavity.

As an example of this approach, curve A of FIG. 8 can be equivalentlysynthesized by the series of short pulses shown in curve B of FIG. 8.

An alternative and innovative approach to synthesizing a power versustime profile of FIG. 5 is to capitalise on the transient behaviour ofthis type of laser to deliberately trigger a series of high frequencysmall pulses, each of peak power well above the steady state continuouswave output of the laser, but well below the normal level of Q-switchedpulses. Such a transient response should be able to be triggered whenoperating in steady state cw mode by introducing optical loss into theoptical resonator for a fraction of a microsecond, long enough toextinguish the lasing action and the corresponding stimulated emission,therefore reducing the photon flux to approximately zero. In thisscenario, N should remain approximately equal to Nph, but graduallyincrease to above this since the generation of excited electrons nowexceeds their recombination. This allows energy to be stored in thelaser rod, with the amount determined by the duration of application ofloss to the optical cavity. If the loss is almost immediately removed,stimulated emission would be allowed to reinitiate, but at a much lowerlevel than steady state due to the much lower photon flux that is waybelow the steady state value. This therefore should allow the gain toremain below unity for a period during which N therefore would continueto increase to a level well above N_(ph) as shown in FIG. 9. Theincreasing N and photon flux should eventually lead to the gainexceeding unity, triggering the generation of a small pulse from thelaser output. Such a pulse will lead to N falling to well below N_(ph),but to a value still well above that existing after normal high energyQ-switched pulses. In this case, the gain falls naturally to below unitywithout the need for the introduction of loss into the optical cavityand so the photon flux does not fall to such a low value. This processthen repeats itself except that the next pulse should be a littlesmaller as shown in FIG. 9 because it will be triggered more quickly dueto the photon flux not needing to build from almost zero. Followingseveral more of these cycles, with each subsequent pulse being a littlesmaller as shown, the laser output relaxes back to a steady stateoutput.

Importantly for the laser doping process, the frequency of these smallpulses during the transient period of its underdamped response issufficiently high that the silicon absorbing the energy does not have achance to appreciably change temperature between the small pulses due tothe silicon's limited thermal conductivity. The silicon thereforeeffectively filters the waveform of FIG. 9 similarly to in Approach 3.What is therefore important is the average energy delivered to thesilicon during the transient period, following which the laser can bemade to operate on continuous wave for at least as long as is necessaryfor the silicon to be molten to facilitate adequate dopant mixing.Provided loss can be introduced into the optical cavity when requiredand for the desired duration, then this in combination with setting thelaser power at the level necessary for the steady state cw operation tomaintain the molten silicon at approximately constant temperature, thisapproach can in theory also be used to control all the important aspectsof the Power versus Time profile of FIG. 5 for the laser doping process.This includes the duration of the overall pulse (for the purpose ofmaintaining the silicon in its molten state for the required period),controlling the amount of energy in the initial high energy transientpart of the pulse (that melts the silicon through control of theduration for which the loss is introduced into the optical cavity) andthe power level during the steady state cw period of the pulse (thatmaintains the silicon at constant temperature while the dopantsadequately mix). No known existing lasers however have this capabilityor make it possible for the user to gain this capability. New circuitryfor controlling the Q-switch has had to be developed for introducing theloss into the optical cavity to allow this flexibility and control bythe user.

Another approach is to use the 532 nm wavelength laser on continuouswave mode to avoid the thermal cycling that exacerbates the defectgeneration, but simultaneously move the laser beam during theheating/melting process so as to control the amount of energy beingdelivered to the silicon being doped in a particular location andtherefore the temperature it reaches. For example, a particular locationbeing laser doped can be heated somewhat without direct exposure to thelaser beam by delivering the laser energy to a nearby location and usingthe thermal resistance of the silicon and the distance away from thelocation, to control how much energy is delivered to the location beingdoped. For example, if it was possible to instantly move the laser beamfrom one location to another location, the power delivered to aparticular location being laser doped could be made to take the form ofFIG. 5 whereby period t₁ represents the initial direct application ofthe continuous wave (cw) laser to the location being laser doped,following which it is relocated to a nearby location for time t₂ wherethe thermal resistance of the silicon reduces the energy delivered tothe location being laser doped as shown in FIG. 5, following which thelaser beam is removed well away so that the energy delivered to thelocation being laser doped is reduced to a negligible value. Although intheory this is feasible, in practice lasers cannot operate quite likethat, firstly because they cannot instantly change location andsecondly, because the laser beam, even when focused has a finitediameter, usually greater than 10 microns which makes it impossible toprecisely deliver power of the form of FIG. 5 to all parts of thesilicon directly exposed to the silicon beam during t₁.

However, by appropriately controlling the steady state power of the cwlaser E_(ss) and then scanning the laser beam at the right speed towardsthe location to be laser doped and then away again from the location atthe right speed, the energy delivered to this particular location cantake the form as shown in the top graph of FIG. 10. As shown in thebottom graph of FIG. 10, this can facilitate the melting of the siliconin this location while also facilitating keeping it molten for at least2 microseconds to enable adequate dopant mixing while simultaneouslyavoiding ablation of the silicon and repeated thermal cycling. Ifhowever the scan speed for the laser is too high, then the silicon maynot even reach the melting temperature at the location being laser dopedor alternatively, the silicon may melt but not remain molten long enoughfor the dopants to adequately mix. At the other extreme, if the laserscan speed is too low for the given laser power E_(ss), then too muchenergy will be delivered to the molten silicon and it will exceedtemperature T_(v) at which the silicon and dopants will be ablated.

Similarly there exists only a certain range of acceptable power levelsfor E_(ss) from the laser for which the laser doping process will beadequately performed. E_(ss) values too low will either fail to melt thesilicon or else necessitate such slow scanning speeds for the laser toachieve melting that the process is not viable. At the other extreme,E_(ss) values too high will necessitate such high laser scanning speedsto avoid ablation of the silicon that the energy delivered to thelocation will fall to too low a value too quickly to keep the siliconmolten for the required 2 microseconds or longer.

For example, using a 13 Watt 532 nm cw laser with a scan speed for thecontinuous wave laser of 3 m/sec and a beam diameter of 12 microns, eachpoint will be directly illuminated by the laser for 4 microseconds. Whenthe laser power is well matched to the scan speed as shown in FIG. 10,the silicon will need to be illuminated by the laser for almost half thetotal direct illumination time to reach the silicon melting point ofabout 1400 degrees C. at time t₂. For this example, this will mean thatthe laser beam will have illuminated the given point for almost 2microseconds before the silicon at that point melts. By the time thedirect illumination ceases (in this case after a further 2 microsecondsat time t₃) the molten silicon will have been molten for at least 2microseconds while being heated to almost 2,000 degrees C., still wellbelow the ablation temperature for silicon. For the next 1-2microseconds, the laser beam, although moving further away from thegiven point, still delivers sufficient heat to the given point to allowits temperature to continue to increase following which it cools quiterapidly, reaching the freezing temperature of 1400 degrees C. for thesilicon at time t₄. Overall it is therefore possible with this approachto avoid ablation of the silicon, avoid thermal cycling of Q-switchedoperation, still keep the silicon molten for at least 2 microseconds,while still using high laser scanning speeds suitable for highthroughput commercial production. Using the same E_(ss) value from the13 Watt laser, scan speeds as low as 2 m/s can be used before ablationof the silicon commences while scan speeds as high as 5 m/s can be usedwhile still keeping the silicon molten for at least 2 microseconds.

Unfortunately laser power levels below about 12 Watt cease to be able tomelt the silicon without unacceptably low scan speeds. Increasing thepower level though to 14 Watts allows a scan speed as high as 10 m/s tobe used. In this case, in FIG. 10, the time period from t₁ to t₂ isabout 1 microsecond followed by a little over 1 microsecond of furtherdirect illumination of the laser until t₃. The silicon temperature peaksat time t₄ following which freezing at t₅ occurs at about 2 microsecondsafter t₂ when the silicon melted. Consequently the conditions aresatisfied for adequate laser doping of the silicon. At this power level,the slowest scan speed acceptable is 3-4 m/s, below which siliconablation commences.

One method for adjusting the laser scan speed to be appropriate for agiven laser power E_(ss) (or visa versa) is to ensure the laser meltsthe silicon when the laser beam has passed approximately halfway overthe point being laser doped. This is equivalent to saying that in FIG.10, for a constant laser scanning speed, the time interval between t₁and t₂ is approximately the same as the time interval from t₂ to t₃.FIG. 11 shows a schematic representing this where the unmelted siliconilluminated by the laser beam for less than the time interval (t₂−t₁)still has surface pyramids while the part of the surface illuminated bythe direct laser beam for longer than (t₂−t₁) has already melted,therefore destroying the pyramids and providing a much flatter and morereflective surface. In FIG. 11, point 4 has not yet been illuminated bythe beam while point 3 has just finished being illuminated and point 5has already been passed by the beam and cooled. For a constant scanspeed laser, best results are achieved when the ratio between thedistance between point 1 and 2 to the distance between the points 2 and3 is within the range 1:3 to 3:1. For practical laser speeds with ratiosbelow 1:3, the silicon near point 3 has been exposed to the laser beamfor too long while in the molten state so that ablation of the siliconcommences. At the other extreme, for ratios above 3:1, the resolidifiedand cooled silicon at point 5 runs the risk of having not been moltenfor more than 1 microsecond to facilitate adequate mixing of thedopants. This provides a relatively simple mechanism to check whetherthe laser scan speed is properly matched to the laser power since thereflection from the surface illuminated by the beam can be measured toindicate what percentage of the illuminated area is still covered bypyramids and therefore not yet melted which in turn indicates whetherthe ratio is within the correct range. If the ratio is too small, thesurface reflection will be too high due to more than 75% of the areabeing illuminated by the beam being quite flat and therefore reflective,while too high a ratio will mean that more than 75% of the area beingilluminated by the beam will be still covered by pyramids making thesurface far less reflective. Various techniques can be used to measurethe surface reflection of the illuminated area for these purposes. Forexample it is common to use reflected light from the illuminated areafor the purposes of viewing the area being processed by the laser.Measuring the intensity of this reflected light such as with aphoto-detector or solar cell will give the required information. Thisinformation can be fed back to the laser controller to allow it toautomatically adjust either the scan speed or the laser power to ensurethe ratio always remains within the correct or desired range.

Referring back to FIG. 10, several conditions for good laser doping canbe identified, for which appropriate values of E_(ss) and scan speedneed to be chosen (in conjunction with each other) to satisfy. Firstly,the maximum temperature reached by the silicon T_(u) must be greaterthan the melting temperature for the silicon T_(m) and lower than thevaporisation temperature T_(v). Secondly, the time period t₅−t₂ forwhich the silicon is molten, must exceed 1 microsecond and preferablyexceeds 2 microseconds. Thirdly, the scan speed for the laser whenachieving the results of FIG. 10 needs to be at least 1 m/s forpractical purposes relating to device throughput. Fourthly, scan speedsin excess of 20 m/s are unsuitable due to failure to keep the siliconmolten for long enough. Fifthly, 532 nm wavelength cw lasers below 10watts provide Ess values too low to allow the silicon to be melted whilesimultaneously achieving adequate scan speeds to be practical. Sixthly,532 nm wavelength cw lasers operated at power levels above 20 Wattsprovide Ess values that are too high, therefore necessitating scanspeeds for the laser that are too fast to allow the molten silicon toremain molten for more than 1 microsecond as required.

For the above 5 approaches, line lasers can in general be used insteadof lasers that illuminate a single point. For a line laser where anentire line rather than a small circle of typically 15 microns diameteris simultaneously illuminated, all the same principles of theseapproaches apply but whereby no Q-switched pulses are generated duringthe time period where the laser is being moved from the location of oneline being laser doped to the location of the next line to be laserdoped. For a continuous wave laser however, the scan speed for shiftingthe laser beam from the location of one line to that of the next has tobe fast enough so that the silicon being illuminated in between does nothave time to reach melting point. In this case the scan speed isvariable whereby the scan speed is reduced approaching the location of agiven line for laser doping and then speeds up when departing thatlocation to move to where the next line is to be located. In this way,varying the scan speed still allows the Energy from the laser to takethe form given in FIG. 10 whereby the scan speed is slow between time t₁and t₃.

Liquid Jet and Gaseous Solutions.

In some embodiments the laser may be enveloped in a liquid jet such thatthe substrate surrounding the point of laser heating is kept cool, andthe liquid can be used to deliver reagents to the point of processingunder the laser.

FIG. 12 schematically illustrates the arrangement of a laser and liquidjet when used to process a substrate provided with a dielectric surfacelayer. In this case the surface layer need not provide a dopant source.As illustrated, a laser 60 emitting a laser beam 61 is projected througha covered window 62 in a nozzle unit 63. A liquid jet 64 is generated bythe nozzle unit into which the laser beam 61 is coupled such that it maybe guided by total internal reflection towards the target. A supply ofliquid to the nozzle unit 63 is provided through ports 65 and isexpelled through a nozzle orifice 66 which projects the liquid towardsthe target. The window 62 is oriented to receive a vertical laser beam61 which is directed axially into the liquid jet 64. The laser beam 61is focussed by appropriate lenses 67 before entry through the window 62.Liquid is delivered to the nozzle unit 63 with a pressure of between 20to 500 bar via the liquid supply port 65. The liquid may be suppliedfrom a reservoir 72 or other suitable source and pumped under pressureto the nozzle unit 63 by supply pump 73. The liquid may also be heatedby heater 74 so that the temperature of the liquid jet may becontrolled. The generated liquid jet 64 may have a diameter in the rangeof approximately 20 to 100 μm.

The liquid jet 66 and laser beam 61 are shown directed to a target whichis a 250 μm silicon substrate 68 with a 30-80 nm thick silicon nitridelayer surface layer 69. The liquid jet 64 and laser beam 61 are guidedover regions of the surface layer as with conventional laser dopingmethods. By adding phosphoric acid to the liquid jet a strong corrosiveaction will take place on the silicon nitride layer and the underlyingsilicon where the surface becomes heated by the laser beam 61 leavingthe surface layer 69 very cleanly and precise ablated, whereas thesubstrate 68 is left substantially intact elsewhere. Additives are addedto the liquid jet 64 from one or more supply tanks 75 and injected byrespective pumps 76 into the portal 77 of the nozzle unit 63. Howevern-type doping of a surface region 71 of the silicon can also beperformed simultaneously with the nitride removal by virtue of thephosphoric acid used for cleaning, or by the inclusion of additionaldopant additives such as POCl3, PCl3, PCl5, or a mixture of these.P-type doping could also be achieved in a similar operation by selectingthe appropriate dopants (e.g. Boron).

The liquid jet 66 and laser beam 61 may also be used with dopant sourcesincluded within a surface dielectric layer 17, sources applied as acoating on top of or below the dielectric layer (also potentially theantireflection coating 16), or the dopant atoms may be present in thesilicon in an unactivated state whereby they are absorbed into thesilicon structure (or activated) by the melting and refreezing processas described above with reference to other laser systems.

Laser operations may also be performed in a gaseous environment toachieve the doped surface region 71 without providing dopant in theliquid jet. Referring to FIG. 13, in this case the laser 60 emits laserbeam 61 which is directed through a window 82 in a chamber 81 containingthe target substrate 68. A dopant source in gaseous form is suppliedfrom a pressurised storage cylinder 85 via control valve 84 and port 83,into the chamber 81. Gas is expelled from the chamber via exhaust port86 and exhaust vale 87 to a disposal passage 88. The laser is scannedover the surface of the substrate 68 as before, melting theantireflection coating 69 and a portion of the underlying surfacewhereby the gaseous dopant is absorbed into the molten silicon surfacematerial to form the doped surface layer 71.

While the use of a gaseous environment is described above in conjunctionwith the use of a laser beam projected within a liquid jet the gaseousenvironment may equally be employed with any of the other laserarrangements described above that are not associated with a liquid jet.Liquid dopant sources may also be employed with any of the laserarrangements described, other than as a liquid jet through which thelaser beam is projected. The liquid source may be pooled or flowed overthe surface which is being doped or may be applied as a jet directed atthe point of laser heating.

Preferred Anti-Reflection Coatings

Embodiments of a multiple layer ARC are described below that can bedeposited in a single process (using a single piece of equipment) toachieve thermal expansion mismatch correction for a high efficiencycommercial solar cell.

The approach adopted is to use a triple layer ARC that can be depositedin a single in-line PECVD, E-beam or sputtering deposition process. Thefirst very thin layer could be silicon nitride and need only be thickenough to provide surface passivation and the hydrogen source forsubsequent passivation of the silicon, but not too thick so as to stressthe silicon surface during thermal cycling. The second layer is expectedto be about 100 to 300 angstroms thickness of a material with thermalexpansion coefficient less than the semiconductor material beingprocessed to ensure the semiconductor surface is placed undercompression rather than tension when at elevated temperatures. Thisprovides relief from the thermal expansion mismatch which wouldotherwise be created between the ARC and the semiconductor material, andwhich leads to defect generation at elevated temperatures due to thesemiconductor surface being placed under tension by the overlying ARC.Importantly, the thickness of this second layer needs to be thin enoughso as to not have significant impact optically. The third and by far thethickest layer is a material such as silicon nitride deposited with theright thickness and refractive index to provide the requiredantireflection optical properties for the ARC.

By way of example, when the semiconductor material from which a deviceis fabricated is crystalline silicon, this material is known to easilysustain defects when the surface is under tension, particularly if atelevated temperature. Silicon nitride, deposited by plasma enhancedchemical vapour deposition (PECVD), is known to do a good jobpassivating the silicon surface, bulk and grain boundaries, primarilydue to the high concentration of atomic hydrogen present in thedeposited silicon nitride that is able to tie up dangling bonds at thesilicon surface, defects or grain boundaries. A thin layer of PECVDsilicon nitride in the range of 10-200 angstroms thick is therefore agood choice for the first layer when the semiconductor is crystallinesilicon.

A good choice for the second layer in this example would be silicondioxide or silicon oxynitride since these have a thermal expansioncoefficient less than that of silicon and can also be deposited by PECVDby appropriately varying the gases and their flow rates. The thicknessof this layer is quite important. If too thick it excessively degradesthe optical properties of the overall ARC since its refractive index isnot well suited to the requirements of the ARC. If too thin, the layeris unable to compensate for the stress created on the silicon surface bythe overlying third layer of the ARC which is significantly thicker.This second layer is at least as thick as the first layer but not asthick as the third layer. With PECVD it is possible to tailor this layerin terms of having a graded composition and refractive index to furtheraid with the optimisation. The inclusion of some nitrogen allows siliconoxynitrides of virtually any refractive index from below 1.5 to above 2to be achieved.

A good choice for the third layer is PECVD silicon nitride which can bedeposited in the same equipment and process as the first two layers. Ithas close to an ideal refractive index, while the thickness is chosen inconjunction with the thicknesses of the first two layers to overall givethe best anti-reflection properties. Typical thicknesses of the threelayers are 100 angstroms for the first layer with refractive index of2.0, 180 angstroms for the second layer with refractive index of1.5-1.6, and 400 angstroms for the third layer with refractive index of2.0. The overall reflection for this multi layer ARC is almost identicalto an ideal SLARC with only about a 1% increase in reflection.

Example 1

The application of the multi-layer ARC to multicrystalline siliconwafers has demonstrated efficiencies in the vicinity of 17% usingstandard commercial grade p-type multicrystalline silicon wafers. Anexample of a suitable fabrication sequence is as follows:

1. Isotropic texturing 12 of the front (or light receiving) surface ofthe p-type wafer 11;

2. front surface diffusion of n-type dopant 13;

3. edge junction isolation/psg removal;

4. four layer ARC deposition on the front surface by PECVD;

a. 100 angstroms of hydrogen rich silicon nitride 14;

b. 180 angstroms of silicon oxynitride 15 of refractive index 1.5-1.6;

c. 400 angstroms of silicon nitride 16 of refractive index 2.0-2.1;

d. Optional additional dopant containing layer 17; (This layer may beused

if dopants are not already included in the already deposited layers andwhere an additional separate dopant layer is not to be appliedsubsequently) with refractive index optically matched to the encapsulantto be used during module formation to alleviate the need to remove thedopant source layer following the laser doping process5. screen-print rear (non-light receiving) surface with aluminium forrear contact 18;6. fire rear surface to sinter rear contacts 18 and form back surfacefield 19 by formation of aluminium/silicon alloy and liquid phaseepitaxy;7. Optional (if step 4d is omitted) application of an n-type dopantsource 21 to front surface (liquid phosphorus or in order to alleviatethe need to remove the source following the laser doping process anotherdopant containing source with refractive index optically matched to theencapsulant to be used during module formation)8. laser doping of silicon in localised regions to form heavily doped(n+) regions 22 for formation of self-aligned front surface metalcontacts;9. Plating a layer of Nickel 23 over the laser doped n+ regions 22 forthe front surface contacts;10. sintering of Nickel 23;11. Plating of layers of Copper 24 and Tin 25 (or Silver) over theNickel 23;

The above processing sequence produces the high performance solar cellstructure of FIG. 1 with a selective emitter that provides heavy dopingof the silicon directly beneath the metal contacts. The four layer ARC(counting the step 4d dopant source) performs well optically, with onlyabout 1% increase in reflection compared to a single layer ARC followingencapsulation whereby the single layer comprises silicon nitride withthe same refractive index as the third layer and with optimal thickness.In terms of defect generation, during the laser doping, the regionsimmediately adjacent to the molten regions are heated to almost 1400degrees Celsius, but encounter minimal defect generation due to the lowthermal expansion of the second layer (deposited in step 4b) avoidingsignificant tension being applied to the silicon surface. The firstlayer (deposited in step 4a) contains sufficient atomic hydrogen tofacilitate surface and grain boundary passivation for themulticrystalline silicon during the deposition process and during theshort high temperature treatment during the firing of the aluminium rearcontact.

Previously large volume commercial manufacture of multicrystallinesilicon solar cells with selective emitters has not been achievable.Even with the benefits of the laser doping techniques for localiseddoping of the silicon, previous attempts at making high performanceselective emitter solar cells have failed for the following reasons:

1. The use of a SLARC has lead to excessive defect generation juxtaposedto the molten regions during the laser doping process due to the highthermal expansion coefficient of the ARC material that places thesilicon surface under tension;

2. The use of an SiO2 layer of optimal thickness instead of the siliconnitride layer is able to avoid the defect generation, but has pooroptical properties due to its low refractive index leading tounacceptably high levels of surface reflection;

3. A thin thermally grown SiO2 layer of typically 180 angstromsthickness prior to silicon nitride deposition has been able to overcomethe problems associated with the thermal expansion mismatch between thesilicon and the silicon nitride, but has introduced three otherproblems. Firstly, it involves an additional process that cannot be donein the PECVD system since such thermally grown oxides need temperaturesin excess of 900 degrees. Secondly, the requirement for such hightemperatures during the SiO2 growth often damages the multicrystallinesilicon wafer. Thirdly, the SiO2 layer acts as a barrier to the atomichydrogen, preventing it from passing from the silicon nitride layer intothe silicon where it is required for passivation of the grainboundaries.

An approach that still incorporates the traditional SLARC techniqueinvolves modifying step 4 of the sequence in the above example asfollows to apply an expansion mismatch correction material layer after aconventional SLARC:

4′. four layer ARC deposition on the front surface by PECVD;

a. 100 angstroms of hydrogen rich silicon nitride (optional);

b. 600 angstroms of silicon nitride having a refractive index of2.0-2.1;

c. 1,000 angstroms of silicon dioxide or silicon oxynitride having arefractive index of 1.5-1.6. Optionally this layer may include dopantfor the laser doping step (step 8). The refractive index may beoptically matched to the encapsulant to be used during module formationto alleviate the need to remove the dopant source layer following thelaser doping process, or optionally the layer may be removed aftercompletion of thermal processing. Note also that 4a. and 4b. above canpotentially be combined in this embodiment to be a single layer ofsilicon nitride of thickness about 700 angstroms.

A variation of the present multi-layer ARC that facilitates zerothickness for the first (ie passivation) layer can be used if certainconditions are met that alleviate the need for such high quality surfacepassivation. These include:

a) firstly, devices that are only required to achieve relatively lowvoltages below 650 mV that therefore do not require the same quality ofsurface passivation;

b) secondly, devices that have a diffused surface that reduces the needfor such a low surface recombination velocity to achieve a given opencircuit voltage for the device;

c) thirdly, devices with a shallow junction of less than 1 micron depthso that the poorer surface recombination velocity compared to when thefirst layer in step 4a is used does not have severe consequences for thecollection probabilities for charge carriers generated from shortwavelengths of light in the vicinity of 300-500 nm; and

d) fourthly, where atomic hydrogen is incorporated into the layerdeposited in step 4b and the layer is made thin enough so as not to actas too severe a barrier to the passage of atomic hydrogen into thesilicon to passivate the grain boundaries or alternatively, a siliconnitride layer is deposited onto the rear surface of the wafer to providethe hydrogen source for the grain boundary passivation.

Another example of the potential use of a multi-layer ARC is in regardto single crystalline silicon material where there is no necessity forgrain boundary passivation. In this case the first layer deposited instep 4a can be significantly thinner since significantly less atomichydrogen is required. By having this layer thinner, it is alsoacceptable to have its refractive index well above that of siliconnitride by making the layer silicon rich without fear of excessiveabsorption of the short wavelengths of light. The higher refractiveindex layer provides superior surface passivation with lower interfacedefect densities which is important in high voltage devices which arefeasible with the higher quality single crystal silicon wafers. In thisimplementation of the multi-layer ARC, the second and third layers stillperform the functions described in the example above and can havedeposition parameters as previously indicated.

In another example of the use of the multi-layer ARC, if thermaldiffusions are used instead of laser doping to produce the heavily dopedregions beneath the metal contacts, the ARC can be used as a diffusionmask. In this case, the third layer for the ARC needs to besignificantly thicker since the dopants also diffuse into the siliconnitride layer as well as the exposed silicon unless the dopants areselectively deposited in localised regions such as by inkjet printing ofthe dopants. In either case, the low thermal expansion coefficient ofthe second layer is again required to prevent the silicon surface frombeing placed under tension during the thermal diffusion process.

It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the invention as shown inthe specific embodiments without departing from the scope of theinvention as broadly described. The present embodiments are, therefore,to be considered in all respects as illustrative and not restrictive.

The invention claimed is:
 1. A method of forming a doped surface regionof a semiconductor material at a surface of a semiconductor solar celldevice during its manufacture, the semiconductor material comprisingsilicon, a dielectric material surface layer extending over the surfaceregion of the semiconductor material and the doping being performed inone or more localised regions of the semiconductor material, the methodcomprising: locally heating a surface of the semiconductor material inan area to be doped to locally melt the semiconductor material to form amolten semiconductor material, the melting being performed in thepresence of a dopant source whereby dopant from the dopant source isabsorbed into the molten semiconductor material, and wherein the heatingis performed in a controlled manner such that a region of the surface ofthe semiconductor material in the area to be doped is maintained in amolten state without refreezing for a period of time greater than onemicrosecond.
 2. The method of claim 1 wherein the dielectric materialsurface layer is provided to perform one or more of the functions of asurface passivation coating, an antireflection coating, or a platingmask.
 3. The method of claim 1 wherein the area to be doped isprogressively doped by sequentially locally heating regions within thearea to be doped.
 4. The method of claim 3 wherein a heating source iscontinuously scanned over the surface of the semiconductor material suchthat the region being heated is continuously moving over the surface,creating a moving melting edge and a molten tail extending from a regionof refrozen doped material.
 5. The method of claim 3 wherein a heatingsource is continuously scanned over the surface of the semiconductormaterial, such that a currently heated region overlaps previously heatedregions whereby heating one region contributes heat to an adjacentpreviously heated region, and to which the source is no longer applied,to reduce a rate of cooling of the previously heated region.
 6. Themethod of claim 5 wherein the heating source has a constant output. 7.The method of claim 3 wherein the heating is applied to discrete regionssuch that a newly heated region will melt and will also contribute heatto an adjacent previously heated region, and to which the source is nolonger applied, to reduce a rate of cooling of the previously heatedregion.
 8. The method of claim 7 wherein, a region is subjected torefreezing and remelting to achieve required level of doping and theremelting occurs no more than 3 times.
 9. The method of claim 8 whereinremelting occurs no more than once.
 10. The method of claim 1 whereinthe intensity of heating is varied over time between a higher level usedto initially melt a region and a lower level used to maintain the moltenstate.
 11. The method of claim 10 wherein the level of heating isdecreased after melting a region until and whereby the region beingheated overlaps an already molten region by less than a predeterminedpercentage.
 12. The method as claimed in claim 1 wherein heating isachieved by scanning one or more a laser beams over the surface of thesemiconductor material such that a local region of irradiance by laserbeam creates a melted region; and scanning the laser over the surfaceprogressively melts a continuous line of surface material and acontinuous wave (cw) laser is used to heat the surface of thesemiconductor material.
 13. The method as claimed in claim 1 whereinheating is achieved by scanning one or more laser beams over the surfaceof the semiconductor material such that a local region of irradiance bythe one or more laser beams creates a melted region; and scanning theone or more laser beams over the surface of the semiconductor materialto progressively melt adjacent such regions to form contiguous group ofoverlapping such regions.
 14. The method as claimed in claim 13 whereinthe one or more laser beams comprise a laser beam of a single Q-switchedlaser which is operated in a pulsed operation in which the laser beam ofthe single Q-switched laser comprises multiple beam pulses each shorterthan a maximum beam pulse duration of the Q-switched laser with the beampulses irradiating overlapping regions of the surface of thesemiconductor material in such a manner so as to simulate the effects ofa cw laser whereby areas of the surface of the semiconductor materialirradiated by adjacent beam pulses are sufficiently close to each otherspatially to ensure a majority of silicon melted by the irradiation ofadjacent beam pulses remains molten between the adjacent beam pulses, asthe laser beam of the single Q-switched laser is scanned over thesurface of the semiconductor material.
 15. The method of claim 14wherein a repetition period of the beam pulses of the Q-switched laseris less than 0.02 microseconds.
 16. The method of claim 15 wherein therepetition period of the beam pulses of the Q-switched laser is constantto simulate an output of a constant irradiance cw laser.
 17. The methodof claim 16 wherein the scanned beam of the Q-switched laser is operatedat a substantially constant average level of irradiance on the surfacebeing heated; and a scanning speed is chosen to melt the surface of thesemiconductor material and to maintain a given point on the surface ofthe semiconductor material in a molten state for a period of at least 1microsecond but no more than 10 microseconds.
 18. The method as claimedin claim 13 wherein an average irradiance level of the one or more laserbeams impinging on the surface of the semiconductor material to locallyheat the surface of the semiconductor material varies with time between:at least a high average irradiance level periodically applied for apredetermined time period t1 to cause melting of unmelted semiconductormaterial on the surface of the semiconductor solar cell device withinthe time period t1; and a lower average irradiance level applied for afurther time period t2 after the time period t1 and the lower averageirradiance level which maintains melted semiconductor material to remainin a molten state for the time period t2.
 19. The method of claim 18wherein the one or more laser beams comprise a continuous or pseudocontinuous wave laser beam produced by a cw laser, or a pseudo cw laser,the continuous or pseudo continuous wave laser beam having an irradiancelevel which is varied cyclically in synchronisation with a scanningspeed of the continuous or pseudo continuous wave laser beam such thatthe average irradiance level of the continuous or pseudo continuous wavelaser beam increases during a melting period and then decreases untilthe continuous or pseudo continuous wave laser beam has movedsufficiently over the surface of the semiconductor material for lessthan a predetermined percentage of the beam to be still exposing moltensemiconductor material.
 20. The method of claim 19 wherein a period of acyclic power variation corresponds to a beam width divided by thescanning speed and the period t1 is 50% of the cyclic power variation orless.
 21. The method of claim 19 wherein the irradiance level of thecontinuous or pseudo continuous wave laser beam is varied by varying afocus of the continuous or pseudo continuous wave laser beam to spreadthe continuous or pseudo continuous laser beam during the period t2. 22.The method of claim 19 wherein the irradiance level of the continuous orpseudo continuous wave laser beam is varied by varying an output energyfor a given output period of the continuous or pseudo continuous wavelaser beam.
 23. The method of claim 22 wherein the one or more laserbeams comprise a laser beam of a cw laser and a variable irradiance isprovided by operating the cw laser to produce a cyclic variable outputhaving a power level during a first period t1 of each cycle which meltsirradiated semiconductor material and a power level during a secondperiod t2 of each cycle that maintains irradiated melted semiconductormaterial in the molten state.
 24. The method as claimed in claim 22wherein the continuous or pseudo continuous wave laser beam is a pseudocontinuous laser beam and a Q-switched laser is used as the pseudo cwlaser to produce the pseudo continuous wave laser beam pulses which varywith time such that each pseudo continuous wave laser beam pulse has ahigh irradiance period t1 and a lower irradiance period t2.
 25. Themethod of claim 24 wherein, during the lower irradiance period t2 ofeach cycle, the laser beam of the cw laser has an average power thatdoes not ablate molten semiconductor material.
 26. The method of claim24 wherein the Q-switched laser produces laser beam pulses in which afirst period t1 of each laser beam pulse has a power level which meltsirradiated semiconductor material and a second period t2 of each laserpulse has a power level that maintains irradiated melted semiconductormaterial in the molten state.
 27. The method of claim 26 wherein thesecond period t2 has a power level that does not ablate moltensemiconductor material.
 28. The method of claim 18 wherein during theperiod t2 the molten semiconductor material is kept at a temperature ofbetween 1414° C. and 3265° C. for at least one microsecond.
 29. Themethod of claim 28 wherein during the period t2 the molten semiconductormaterial is kept at the temperature of between 1414° C. and 3265° C. for2-10 microseconds.
 30. The method of claim 18 wherein the one or morelaser beams comprise a laser beam of a single Q-switched laser which isoperated in a pulsed operation in which the laser beam of the singleQ-switched laser comprises multiple beam pulses each shorter than amaximum beam pulse duration of the Q-switched laser with the beam pulsesirradiating overlapping regions of the surface of the semiconductormaterial in such a manner so as to simulate the effects of a cw laser, arepetition period of the beam pulses being less than 0.02 microsecondsand a variable average irradiance is provided by varying the repetitionperiod to produce a cyclic variable laser output, a higher averageirradiance level being produced by providing a shorter repetition periodof the beam pulses and the cyclic output having an average irradiancelevel during a first period t1 of each cycle which melts irradiatedsemiconductor material and an average irradiance level during a secondperiod t2 of each cycle that maintains irradiated melted semiconductormaterial in the molten state.
 31. The method of claim 30 wherein, duringthe second period t2 of each cycle, the laser beam pulses of theQ-switched laser have an average output irradiance level that does notablate molten semiconductor material regardless of the period ofexposure.
 32. The method as claimed in claim 18 wherein laser beams frommore than one laser are used to heat the surface of the semiconductorsurface such that a laser beam of one laser is used to melt the surfaceof the semiconductor material and a laser beam of a second laser is usedto maintain the surface region of the semiconductor material at atemperature to remain molten for a period greater than 1 microsecond toallow the dopant to be absorbed and distributed throughout the moltenregion.
 33. The method of claim 32 wherein Q-switched lasers are used toheat the surface of the semiconductor material.
 34. The method of claim32 wherein a first, Q-switched, laser is used to melt the semiconductormaterial with a beam pulse rate and beam scanning speed selected toallow overlapping exposed areas for consecutive beam pulses.
 35. Themethod of claim 34 wherein the beam pulses of the Q-switched laser areof an intensity whereby the semiconductor material is melted with asingle laser beam pulse and the beam pulses have a duration of less than10% of the beam pulse repetition period.
 36. The method of claim 35wherein an area of irradiation of each beam pulse is overlapped with anarea of irradiation of the previous beam pulse on the surface of thesemiconductor material by 10-50% of an exposure area of the beam pulses.37. The method of claim 34 wherein a laser beam of a second laser isused to ensure that the semiconductor material remains molten betweenthe beam pulses of the first laser.
 38. The method of claim 37 whereinthe second laser is a cw laser operated with a constant output beamintensity.
 39. The method of claim 37 wherein the second laser is aQ-switched laser which is operated to produce output beam pulses inperiods between the beam pulses of the first laser and the second laseris operated at a lower beam pulse irradiance than the first, Q-switched,laser to extend a time during which semiconductor material melted by thefirst laser remains molten.
 40. The method of claim 39 wherein thesecond laser is operated in a beam pulsed operation in which it emitsmultiple beam pulses each shorter than a maximum beam pulse duration ofthe Q-switched laser, and the second laser is scanned at a rate suchthat successive beam pulses of the second laser irradiate overlappingregions of the semiconductor material, the repetition period of the beampulses of the second laser being less than 0.02 microseconds, and therepetition period of the beam pulses of the second laser being constant,to simulate a constant output cw laser at least during the periodbetween the beam pulses of the first laser.
 41. The method as claimed inclaim 1 wherein an area to be doped comprises an entire surface of thesemiconductor material to create a surface emitter layer.
 42. The methodas claimed in claim 1 wherein an area to be doped comprises a part of anentire surface of the semiconductor material where contacts are to beformed on a light receiving surface of a solar cell.
 43. The method asclaimed in claim 1 wherein the dielectric layer carries dopant atoms toperform as the dopant source.
 44. The method as claimed in claim 1wherein the dopant source is a coating which carries dopant atomslocated on top of or below the dielectric layer.
 45. The method asclaimed in claim 1 wherein dopant atoms are present in the silicon in anunactivated state to perform as the dopant source whereby the dopantatoms are absorbed into the silicon structure when the molten siliconcrystallizes.
 46. The method as claimed in claim 1 wherein the dopantsource is in a gaseous state and the heating is performed in anenvironment containing the gaseous dopant source.
 47. The method asclaimed in claim 1 wherein the dopant source is in a liquid state andthe heating is performed while the liquid dopant source is applied tothe heated surface.
 48. The method as claimed in claim 1 wherein thesurface of the semiconductor material is maintained in a molten statefor a period which permits substantially uniform distribution of thedopant throughout the molten semiconductor material.
 49. The method asclaimed in claim 1 wherein the surface of the silicon material ismaintained in a molten state for a period which permits all of themolten region to achieve the same overall dopant polarity.